A diffraction grating is generally a one-dimensional periodic array of protrusions having a triangular or rectangular cross-sectional shape. Depending on the purposes, a two-dimensional periodic array of protrusions or recesses in a pyramidal or rectangular parallelepiped shape also is used as a diffraction grating. Diffraction gratings are classified roughly into two types, a reflective type and a transmission type.
FIG. 17 is a schematic cross-sectional view illustrating one example of a conventional diffraction grating 101. When the period of the grating is greater than half the wavelength of incident light 103, generally diffracted lights 104 and 107 are produced on the reflection side and the transmission side. The angles of these diffracted lights are determined by the wavelength and incident direction of light and the period of grating, so even the light waves that are incident from the same direction can result in diffracted lights in different directions depending on their wavelengths. This principle is utilized in splitting white light into spectra, detecting only the intensity of light with a predetermined wavelength by a light detection device placed in a predetermined direction, and so forth. In a reflective diffraction grating, a metal film is coated on the surface, and therefore, light cannot proceed through to the transmission side. In a transmission diffraction grating, a surface layer 102 for reflecting light is omitted or it is subjected to an anti-reflection coating.
Conventional diffraction gratings have employed the technology of processing its cross section in an appropriate sawtooth shape to attain high diffraction efficiency. As illustrated in FIG. 17, incident light 103 is divided into reflected lights 105 and refracted lights 106 at a slope of one triangle. In the case of a reflective diffraction grating, the inclination angle and period of the slopes are determined so that the reflected diffraction light with a wavelength that is required to be diffracted efficiently can proceed in the direction coinciding with that of the reflected light. In the case of a transmission diffraction grating, its design is conducted so that the direction of desired transmitting diffraction light coincides with that of refracted light. This optimization of the cross-sectional shape for obtaining high diffraction efficiency is called blazing, and a diffraction grating that is optimized in this way is called a blazed diffraction grating.
The blazing principle discussed above, however, can be applied only to the diffraction grating with a period considerably greater than the wavelength because it utilizes geometrical optical phenomena such as reflection and refraction. This type of diffraction grating is called a diffraction grating in the scalar domain. The diffraction grating in the scalar domain may be satisfactory in the case of using a very high diffraction order or in the case where only a very small angle of diffraction is necessary; however, when a low order and a large angle of diffraction are desired, the period and wavelength should be designed to be close values so as to be different by several times at most. This type of diffraction grating is called a resonance domain diffraction grating. The resonance domain refers to a domain in which the ratio of grating period p to wavelength λ is greater than 1 but less than 10 (1<p/λ<10). Unlike for the scalar domain (p/λ>10), no clear design theory of blazing has been offered for the resonance domain. For this reason, resonance domain diffraction gratings are designed by solving Maxwell's equations as rigorously as possible to search for a desirable cross-sectional shape.
Fabricating a diffraction grating that is blazed as designed has not yet been so easy to date, even for the one in the scalar domain with a large period. In every age, the best precision processing technology at the time has been employed for the fabrication of diffraction gratings, and consequently, diffraction gratings always have been expensive elements that can be manufactured only by exclusive people. In earlier times, precision processing machines called ruling engines were used, and such equipment that can produce high-quality diffraction gratings was limited even in the world. Although many of them have been replaced with optical interference exposure techniques, highly sophisticated techniques such as special ion etching and precision replication are required for achieving accurate blaze shapes, and the manufacturers that have such techniques are still limited.
JP 2001-91717A discloses a diffraction grating in which microspheres are stacked to form a close-packed structure. Light is made incident on this diffraction grating so that the Mie resonance condition in each sphere and the Bragg condition originating from the periodic structure of the spheres can be satisfied at the same time. The publication describes an example in which light is incident from the direction −48° inclined from the direction normal to a layer (z-axis direction) toward a close-packed array direction in a plane of microspheres (a y-axis direction, for example, in the later-described arrangement shown in FIG. 1). This diffraction grating is obtained by stacking microspheres in a self-assembled manner and can be fabricated relatively easily. The light diffraction utilizing Mie resonance, however, does not yield high diffraction efficiency.
What has been especially inconvenient in using conventional diffraction gratings is the lack of flexibility of the blazing condition. Once the incident direction, diffraction direction, period, required wavelength, and required diffraction order are determined, the appropriate blazing shape can be determined easily. However, when a diffraction grating is used as an optical spectroscope in particular, the diffraction grating is, for example, rotated with respect to the incident light and it must be used even in a condition that falls outside the blazing condition. For this reason, in designing an optical spectroscope, there has been no other option but to limit its use to a specific wavelength as a typically used wavelength, so it has been only within a certain operational range around the specified wavelength for which high efficiency can be guaranteed.